In recent years, the technological development and demand for electronic devices including portable devices are rapidly increasing and thus demand for secondary batteries as an energy source are rapidly increasing.
In addition, toy model helicopters, drones (such as military unmanned airplanes which can fly and control due to the induction of radio waves), electric vehicles, etc., also need batteries such as rechargeable batteries which are light, small in size, and with high capacity.
Among these secondary batteries, lithium secondary batteries having a high energy density, an operating potential characteristic, a long cycle life, and a low self-discharge rate are commercially available and widely used.
So far, the positive electrode of the lithium secondary battery is made into a very small thickness via a heat treatment and pressing process, after coating a small amount of a positive electrode active material of a slurry state on a thin aluminum (Al) foil of a two-dimensional structure.
If the active material is coated thicker, movement of electrons and lithium ions may be limited and the active material may fall off from the foil. That is, since only a very thin active material can be coated to avoid the limited movement of electrons and lithium ions and the fall-off of the active material, active material, a considerable wide area and weight is required in order to have a high capacity.
Korean Patent Application Publication No. 10-2013-0043750, disclosed a method of producing a metal foam for a secondary battery electrode in which an active material is coated on a metal form with a gel (sol-gel) method, the method including the steps of: (a) preparing a solution containing a precursor compound; (b) penetrating the solution into the surface and the interior of a metal foam; (c) drying the metal foam; and (d) heat treating the metal foam.
In this Korean Patent Application Publication No. 10-2013-0043750, a lithium secondary battery is manufactured by directly coating the active material on the surface and internal pore walls of the metal foam by using a sol-gel method, and thus the active material is thinly coated on the surface and internal pore walls of the metal foam due to flow of the active material in a sol solution, to thereby cause a limit to increase the capacity.
Furthermore, the active material is coated only on the surface of the metal foam in the case of a physical vapor deposition (PVD) method, a chemical vapor deposition (CVD) method, and a thermal spraying method, to thereby cause a limit to increase the capacity.
Further, a precursor solution can be also coated in the pores of the metal foam through an ion plating process, a sol-gel process, a coprecipitation method, an impregnation method, or the like, or a method of forming a coating layer may be also used through drying and firing processes by penetrating a colloidal solution into the pores of the metal foam in which active material particles to be coated are dispersed in the colloidal solution. However, even in this case, the active material is thinly coated on the internal pore walls of the metal foam, to thereby cause unstable cell characteristics.